ci: Build sdk-platform-java before running showcase tests

Author: lqiu96

.github/workflows/dependency_compatibility_test.yaml

@@ -20,6 +20,10 @@ jobs:
           java-version: '21'
           distribution: 'temurin'
           cache: maven
+      - name: Install sdk-platform-java's modules
+        # gapic-generator-java requires Java 8 and is irrelevant for this CI
+        run: mvn -q -B -ntp install --projects '!gapic-generator-java' \
+              -Dcheckstyle.skip -Dfmt.skip -DskipTests -Dclirr.skip -T 1C
       # Run in the root module which should test for everything barring showcase
       - name: Perform Dependency Compatibility Testing
         run: ./.github/scripts/test_dependency_compatibility.sh dependencies.txt